I recently bought Frenzic, which I used to see people list in their favorite casual game lists, as it finally dropped to .99. Its awesome and deserves the hype (that it once had).
TNA, but no one even talked about it in the first place, so I don't think that fits. So then Doom Resurrection will be it for me.
Karmastar - Great graphics and gameplay (sort of a mix between Risk and the Game of Life). I'm really surprised this game never took off.
I'm playing those right now! Isotope is one that I still play though not many people seem to, since the daily leaderboards usually have less than 7 people on them.